Cool-season vs warm-season grasses: which do you have
Almost every UK lawn is cool-season; in the US it depends on latitude. How to tell them apart, and why it inverts every piece of timing advice.
Published 9 February 2026 ·Last reviewed 20 August 2026
Almost every piece of lawn timing advice depends on which of two groups your grass belongs to, and most advice online does not say which it assumes. Getting this wrong means doing the right thing at exactly the wrong time of year.
The difference
Cool-season grasses grow best at around 15–24°C air temperature. They grow hardest in spring and autumn, slow down in summer heat, and stay green through mild winters. Perennial ryegrass, the fescues, Kentucky bluegrass (smooth-stalked meadow grass) and the bents are all cool-season.
Warm-season grasses grow best at around 27–35°C. They grow hardest in summer and go dormant and straw-brown when it gets cold. Bermuda, zoysia, St Augustine, buffalo and centipede are warm-season.
The two curves barely overlap. Advice tuned to one is close to inverted for the other.
Which do you have
In the UK and Ireland: cool-season, essentially without exception. The climate does not suit warm-season grass. A typical UK lawn is a ryegrass and fescue mix, sometimes with bent and meadow grass. If yours goes brown in a drought and greens up when it rains, that is a cool-season lawn under stress, not a warm-season one going dormant.
In the US and Canada it depends on latitude. Roughly north of 37°N is cool-season country. Roughly south of 33°N is warm-season country. Between them is the transition zone, where both are grown and neither is entirely happy.
The simplest test is winter colour. A lawn that stays green through winter is cool-season. A lawn that turns uniformly straw-brown from the first hard frost and stays that way until late spring is warm-season, and that is dormancy, not death.
Why it changes everything
Seeding. Cool-season grass is sown in late summer or spring, into soil around 8–13°C. Warm-season grass is planted in late spring into much warmer soil, and several types are not usually sown from seed at all.
Feeding. Cool-season lawns are fed in spring and autumn, avoiding high summer. Warm-season lawns are fed through summer, which is their growing season.
Mowing. Cool-season mowing peaks in spring and autumn; you may barely mow in August. Warm-season mowing peaks in July and stops entirely in winter.
Watering in summer. A cool-season lawn browning in a drought is dormant and will usually recover; you can let it. A warm-season lawn browning in summer is genuinely struggling, because that should be its best season.
The transition zone
Between about 33°N and 37°N — Kentucky, Tennessee, Virginia, the Carolinas, Oklahoma, northern Arkansas — both types are grown and both have a bad season. Cool-season grass suffers in summer heat and humidity; warm-season grass is brown for a long winter.
There is no clean answer there, which is why lawns in the transition zone are often mixed, and why timing advice written for either end fits badly.
What Mowdar assumes
Mowdar models cool-season grasses only. The growth curve, the germination thresholds and the temperature penalties are all built around them.
That is why it covers the UK, Ireland, the northern US and Canada, and why it declines to answer for locations further south rather than giving a confident answer built on the wrong plant. In the transition zone it says so and lets you decide.
If you have a warm-season lawn, nothing here applies — and it is better that we say so than quietly give you spring advice for a plant that wants July.